Implantable medical devices (IMDs) have been widely used for more than 40 years in various surgical applications. For example, in fracture fixation operations, IMDs are used to address bone fractures by attaching a reinforcing rod or a plate or a cage to a fractured bone so that the broken ends may be stabilized to promote fusion and consequent healing. In sport medicine area, IMDs are used to repair and augment soft tissues, such as anterior cruciate ligament (ACL) replacement. IMDs such as screws are used to affix autografts, allografts, xenografts, or bone fragments to bone structure of a patient. In the case of ACL procedure, the torn ACL is replaced by inserting an IMD in the form of interference screw into a bone tunnel to secure one end of a replacement graft in place.
Metal implants have often been used because of their high stiffness and strength. However, several issues still remain. Metal implants, being much stiffer than bone, become the primary load-bearing member thereby protecting the bone from stress, which results in undesirable stress shielding. It is often necessary to perform a second surgical procedure to remove metal implants after the bone tissues have healed.
The use of biodegradable materials, materials that degrade in the body and then are either absorbed into or excreted from the body, has the potential to eliminate the necessity of a second operation and help alleviate the negative effects of stress shielding. Biodegradable polymeric materials have been used as IMDs in the form of pins, rods, anchors, screws, staples, and fasteners for a variety of medical applications. However, relatively low stiffness and strength of biodegradable devices compared with metallic implants has limited their use to low-load bearing applications or non-load bearing applications.
Inorganic fillers have been used as reinforcement to enhance the mechanical properties of biodegradable polymeric materials. Homopolymers and copolymers of L-lactic, DL-lactic and glycolic acids reinforced with tricalcium phosphate ranging from 0 to 40 percent by weight were studied. Typically, composites of this nature exhibit increased stiffness, but are characteristically brittle.
In U.S. Pat. No. 6,165,486 (to Marra, et al.), hydroxyapatite (HA) granules were incorporated into the blends of poly(caprolactone) and poly(D,L-lactic-co-glycolic) acid for replacing, augmenting or serving as a substitute for hard tissue such as bone. Marra teaches that HA in the range of about 0 to 25 weight percent can be incorporated into the composition. Marra also states, “If the tissue being replaced, augmented, or substituted or the device being formed does not benefit from incorporation of a mineralized component, it is advisable to substantially omit hydroxyapatite from the blend. This is because incorporation of hydroxyapatite results in a more “brittle” device. Minimal or no hydroxyapatite is desirable where a brittle characteristic renders the device or article less useful e.g., sutures, anchors, fixation systems such as sutures, suture anchors, staples, surgical tacks, clips, plates and screws. It is also advisable to avoid large concentrations (i.e., above 10% by weight) of hydroxyapatite soft tissue applications such as tissue used to substitute or augment breast tissue.”
Due to the inherent brittleness and lower strength of these materials, ceramic filler-reinforced biodegradable polymers have often been used in non- or low-load bearing applications such as bone filler or cement.
In another aspect of medical procedures, the movement of a surface of an implantable device with respect to tissue is important in reducing damage to both the material of the surface and to the tissue. Damage to tissue as a result of this tissue-drag friction causes inflammation and pain to the patient and leads to a longer recovery time. High friction between the surface material and blood may result in clotting and subsequent occlusion of a blood vessel. Friction may also damage the material, thus rendering it ineffective or shortening its useful life.
In summary, the inherent brittleness and high stiffness of biodegradable composite systems know to date have limited their usefulness for applications in certain IMDs. In addition, there is a need for methods of reducing device drag in biodegradable composite IMDs while maintaining the degradable nature of the devices.